Fuel cells convert gaseous fuels (such as hydrogen, natural gas, and gasified coal) via an electrochemical process directly into electricity. A fuel cell operates like a battery, but does not need to be recharged and continuously produces power when supplied with fuel and oxidant, normally air. A typical fuel cell consists of an electrolyte (ionic conductor, H.sup.+, O.sup.2-, CO.sub.3.sup.2- etc.) in contact with two electrodes (mainly electronic conductors). On shorting the cell through an external load, fuel oxidises at the anode resulting in the release of electrons which flow through the external load and reduce oxygen at the cathode. The charge flow in the external circuit is balanced by ionic current flows within the electrolyte. Thus, at the cathode oxygen from the air or other oxidant is dissociated and converted to oxygen ions which migrate through the electrolyte membrane and react with the fuel at the anode/electrolyte interface. The voltage from a single cell under load conditions is in the vicinity of 0.6 to 1.0 V DC and current densities in the range 100 to 500 MAcm.sup.-2 can be achieved.
Several different types of fuel cells are under development. Amongst these, the solid oxide fuel cell (SOFC) is regarded as the most efficient and versatile power generation system, in particular for dispersed power generation, with low pollution, high efficiency, high power density and fuel flexibility.
Single fuel cells are connected via interconnects to form multi-cell units, termed fuel cell stacks. Gas flow paths are provided between the interconnects and respective electrodes. Numerous SOFC configurations are under development, including the tubular, the monolithic and the planar design. The planar or flat plate design is the most widely investigated. In this concept the components--electrolyte/electrode laminates and interconnect plates, which may have gas channels formed therein--are fabricated individually and then stacked together and sealed with a high temperature sealing material to form either a fixed or sliding seal. With this arrangement, external and internal co-flow, counter-flow and cross-flow manifolding options are possible for the gaseous fuel and oxidant.
Apart from good electrical, electrochemical, mechanical and thermal properties, the individual cell components must be stable in demanding fuel cell operating environments. SOFCs operate in the vicinity of 950-1000.degree. C. although substantial efforts are under way to reduce the operating temperatures to 800-900.degree. C. For fuel cells to be economical, typical life times of 5-6 years of continuous operation are desired. Thus long term stability of the various cell components is essential. Only a few materials are likely to fulfil all the requirements. In general, the high operating temperature of SOFCs, the multi-component nature of the fuel cell and the required life expectancy of several years severely restricts the choice of materials for cells and manifold components.
A typical solid oxide electrolyte material used in an SOFC is Y.sub.2 O.sub.3 -doped ZrO.sub.2 which is an oxygen ion conductor. However, many other materials have been proposed, and the invention is applicable to all of these. A variety of different anode materials have been proposed for use at the fuel side of SOFCs, but the particular anode material is not relevant to the present invention. However, nickel-containing anodes are preferred. Such anodes have included nickel plating layers and nickel alloys, but the presently most preferred material is a Ni/ZrO.sub.2 cermet. Likewise, a variety of different cathode materials have been proposed for the air side of SOFCs, but the particular cathode material is not relevant to the present invention. However, the presently preferred cathode material is strontium doped lanthanum manganite (LaMnO.sub.3) (LSM).
The purpose of the interconnect between individual fuel cells, as well as at each end of a fuel cell stack and at each side of a single fuel cell, is to convey electrical current away from the fuel cell and/or between adjacent fuel cells and heat away from the fuel cell or cells. To this extent an interconnect should have a relatively high electrical conductivity, which is preferably only electronic or at least primarily electronic, to minimise voltage losses, with negligible contact resistance at the interconnect/electrode interface. It should also have a relatively high thermal conductivity to provide improved uniformity of heat distribution and to lower thermal stresses. A thermal conductivity above 25 W/m K is desirable. In addition, since an intermediate interconnect in a fuel cell stack extends between the anode of one fuel cell and the cathode of the adjacent fuel cell, the interconnect must be impervious to gases in order to avoid mixing of the fuel and the oxidant. Thus, it should have a relatively high density with no open porosity, as well as stability in both oxidizing and reducing environments at the operating temperature. The interconnect should also have high creep resistance so that there is negligible creep at the operating temperature, and a low vapour pressure. The interconnect should further have phase stability during thermal cycling, a low thermal expansion mismatch between cell components, as well as chemical stability with respect to components with which it is in contact. The interconnect should also preferably have reasonable strength, since it may provide structural support, as well as low cost, ease of fabrication and low brittleness.
Ceramic, cermet and alloy interconnects have been proposed. Metallic materials have the advantages generally of high electrical and thermal conductivities and of being easier to fabricate. However, stability in a fuel cell environment, that is high temperatures in both reducing and oxidizing atmospheres, limits the number of available metals that can be used in interconnects. Most high temperature oxidation resistant alloys have some kind of built-in protection mechanism, usually forming oxidation resistant surface layers. Metallic materials commonly proposed for high temperature applications include, usually as alloys, Cr, Al, and Si, all of which form protective layers. For the material to be useful as an interconnect in solid oxide fuel cells, any protective layer which may be formed by the material in use must be at least a reasonable electronic conductor. However, oxides of Al and Si are poor conductors. Therefore, alloys which appear most suitable for use as metallic interconnects in SOFCs, whether in cermet or alloy form, contain Cr in varying quantities.
Cr containing alloys form a layer of Cr.sub.2 O.sub.3 at the external surface which provides oxidation resistance to the alloy. The formation of a Cr.sub.2 O.sub.3 layer for most electrical applications is not a problem as it has acceptable electrical conductivity. However, for solid oxide fuel cell applications, a major problem appears to be the high vapour pressure and therefore evaporation of oxides and hydroxyoxides of Cr (Cr.sup.6+) on the cathode (air) side of the fuel cell at the high operating temperatures. At high temperatures oxides and hydroxyoxides of Cr (Cr.sup.6+) are stable only in the gas phase and have been found to react with the air electrode material leading to the formation of new phases and to deposits of Cr.sub.2 O.sub.3 on the electrolyte. These very quickly reduce electrode activity to the oxygen reduction reaction at the electrode/electrolyte interface and thereby considerably degrade the electrochemical performance of the cell.
It has been attempted to alleviate this problem of degraded electrochemical performance by coating the air electrode side of the interconnect with LSM, which may be the material of the air electrode, but while short term performance is maintained there is a long term degradation which is not acceptable.